This invention relates to freeze indicators. More particularly, it relates to freeze indicators which may be adjusted to provide precise information to the user that a product has been exposed to a certain low temperature normally near the freezing point of water.
Freeze indicators which utilize the expansion characteristic of water to break a frangible ampule have been provided as, for example, in Smith U.S. Pat. No. 3,545,400. Once the device shown in the Smith patent is exposed to temperatures below the freezing point of water, the water expands into ice, causing the ampule to break. After the ice was formed and after the surrounding temperature returned to a point above the melt point of the ice, the water was absorbed on a dye loaded pad, thus giving an indication that the device has gone through a freeze stage and back through a thaw stage.
Problems have arisen in giving an accurate indication of the passage of the device through the normal freezing point of water, i.e. 0 .degree. C., due to the undercooling effect of water which will permit water to stay in its liquid state substantially below its normal freezing point as, for example, as low as -16.degree. C. This problem has been partially overcome by the addition of certain nucleating agents to the water. An example of this is shown in British Patent No. 1,245,135, issued to Scheller. Scheller discloses the technique of adding powdered glass to an ammonium chloride solution to avoid undercooling.
U.S. Pat. Nos. 3,956,153 and 3,980,581, issued respectively to Chadha and Godsey, disclose the use of nucleating agents having substantially similar space groups to thermal responsive materials used in disposable thermometers.